It is common to drive a generator, compressor, pump or other process machine with a prime mover, such as an internal combustion engine. Most internal combustion engines have a speed, or range of speeds, at which they run most efficiently, which is normally measured in revolutions per minute. Also, most engines are designed and built to rotate in one direction only, typically counter-clockwise when facing the flywheel.
In many cases, the most efficient speed of the engine is different than the rotational speed required by the process machine. In other cases, it is not practical to run the engine at the speed required by the process machine. In such cases, it is beneficial to increase or decrease the speed of the engine with external gearing, rather than to adjust the running speed of the engine. It should also be noted that most process machines are designed and built to rotate in one direction only, very often in the same direction as the engine used as the prime mover (typically clockwise when facing the input shaft of the process machine).
A combination of gearing that increases or decreases the speed of an engine's output shaft may be referred to as a speed changer assembly. A speed changer assembly may be disposed externally, between the engine and the process machine, or alternatively internally, as an integrated component of either the prime mover or the process machine.
The ratio of the rotational speed of the output of the prime mover to the rotational speed of the input shaft of the process machine is referred to as the speed (or transmission) ratio of the speed changer. Most speed changers installed in industrial applications have a fixed speed ratio. Change of this ratio typically involves costly, heavy replacement components, use of specialized tools and skilled work-force. It may involve extended down-time of the equipment and is generally regarded as a very expensive option. As such, ratio change is only done when absolutely necessary, which leads to equipment frequently running inefficiently, at less than optimum conditions.
The speed ratio requirement in the speed changer is calculated based on parameters of prime mover, process machine and processed media (gas, fluid, slurry etc.). For initial installations, these calculations are often based on input data that may be unreliable such that change of speed ratio is often desirable shortly after the start-up of equipment. Also, “field conditions” often change during the lifetime of any given installation which, again, leads to the need for a change of speed ratio, which must often be done “in the field”.
There is a need for a speed changer that uses single-speed gearing for maximum efficiency. Moreover, the single-speed gearing arrangement needs to be designed such that the speed ratio change can be effectuated with less difficulty and with low-cost replacement components. Effectuating the ratio-change should be practical in “field-conditions” with no or minimum requirement for special tooling, without the need for highly specialized skilled work-force. Simple, inexpensive speed ratio change of a speed changer may provide possibilities for better optimization of equipment leading to higher overall efficiencies and lower life-cycle cost of the equipment.